Ca2+/calmodulin-dependent kinase II (CaMKII) is a multifunctional protein kinase activated by the complex of Ca2+ and calmodulin. CaMKII mediates phosphorylation of a wild range of target proteins involved in a multitude of cellular processes such as Ca2+ handling, cell growth, and cell death. The d isoform of CaMKII family is predominantly expressing in cardiac myocytes. There are, at least, two splicing variants of CaMKII-d, dB and dC, located in unclear and cytosol compartments, respectively. Our previous studies have shown that enhanced CaMKII-dC activation is both necessary and sufficient for a1AR-induced cardiomyocyte apoptosis, in addition to its well-established functions in regulating phosphorylation of cardiac Ca2+ handling proteins and thus modulating cardiac excitation-contraction coupling. However, the functional role of CaMKII-dB remains elusive, although it has been implicated in cardiac hypertrophy.[unreadable] The aim of the present study is to investigate the potential physiological and pathological functional roles of CaMKII-dB in the heart and explore its clinical implications. We have demonstrated that CaMKII-dB expression is remarkably attenuated at both mRNA and protein levels in rat ischemia/reperfusion (I/R) and myocardium infarction (MI) models and in cultured cardiac myocytes in response to oxidative stress with H2O2. The inhibitory effects of MI and H2O2 on CaMKII-dB are fully prevented by reactive oxygen species (ROS) scavengers, indicating ROS constitutes a negative regulator of CaMKII-dB gene expression. Concurrently, MI and H2O2 markedly increase myocyte apoptosis in vivo and in culture, respectively, assayed by DNA laddering, Hoechst or TUNEL staining, and caspase activation. Most importantly, overexpression of CaMKII-dB using adenoviral gene transfer substantially protects heart cells against ischemia- or oxidative stress-induced apoptosis. Moreover, several lines of evidence indicates that the CaMKII-dB cardiac protective effect is mediated by a Akt-dependent pathway. First, overexpression of CaMKII-dB leads to a robust elevation in Akt phosphorylation. Second, increased CaMKII-dB activation also profoundly augments the expression of a novel apoptosis repressor, ARC (apoptosis repressor with caspase recruitment domain), via a Akt-mediated mechanism, because inhibition of the PI3K/Akt signaling pathway with LY 294002 can substantially block CaMKII-dB induced upregulation of ARC and the protective effect. These in vivo and in vitro data indicate that CaMKII-dB plays a pivotal role in protecting the heart against ischemia and oxidative insults through an Akt and ARC-dependent pathway, marking CaMKII-dB as a promising novel therapeutic target for the treatment of myocardial ischemic disease, the predicted number killer in the world by 2020.